[3.1.x] Fixed #31863 -- Prevented mutating model state by copies of model instances.

Regression in bfb746f983.

Backport of 94ea79be13 from master

django/db/models/base.py

@@ -544,7 +544,10 @@ class Model(metaclass=ModelBase):
 
     def __getstate__(self):
         """Hook to allow choosing the attributes to pickle."""
-        return self.__dict__
+        state = self.__dict__.copy()
+        state['_state'] = copy.copy(state['_state'])
+        state['_state'].fields_cache = state['_state'].fields_cache.copy()
+        return state
 
     def __setstate__(self, state):
         pickled_version = state.get(DJANGO_VERSION_PICKLE_KEY)

tests/model_regress/tests.py

@@ -1,3 +1,4 @@
+import copy
 import datetime
 from operator import attrgetter
 
@@ -256,3 +257,17 @@ class EvaluateMethodTest(TestCase):
         dept = Department.objects.create(pk=1, name='abc')
         dept.evaluate = 'abc'
         Worker.objects.filter(department=dept)
+
+
+class ModelFieldsCacheTest(TestCase):
+    def test_fields_cache_reset_on_copy(self):
+        department1 = Department.objects.create(id=1, name='department1')
+        department2 = Department.objects.create(id=2, name='department2')
+        worker1 = Worker.objects.create(name='worker', department=department1)
+        worker2 = copy.copy(worker1)
+
+        self.assertEqual(worker2.department, department1)
+        # Changing related fields doesn't mutate the base object.
+        worker2.department = department2
+        self.assertEqual(worker2.department, department2)
+        self.assertEqual(worker1.department, department1)